This can only be a series I. The other possibilities were:
no.4 Petzval "objectif a monture universelle" - this is the series with an extra flange thread on the brass hood! EFL is 24cm.
no.5 Petzval "ordinaire" from the early Wallet et Hermagis period late 1840's. EFL is also just 24cm, too.
Nos. 4, 5 and 6 series II Petzvals from ca. 187. These have EFLs of 22, 19 and 16cm, respectively.
So it must be the most common series I Petzval which is:
no.5 EFL
31cm so I think your measure is a little off!
There is a recent thread with the identification of exactly this lens, so the OP can measure his barrel length.
http://www.largeformatphotography.in...light=hermagis
This thread includes an extract from a late Hermagis catalogue on the series I. The number 5 size would have been a common choice for portrait studios.
Measuring the EFL is difficult when there is no Waterhouse, so comparison whith a lens of know focal length (compare length shown on the ground glass - or white wall) is best.
I may have the lens set too in the form of the "universelle" version, but I need to check the exact EFL! My barrel is 15.10cm
from end to end.
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